1. Technical Field
This disclosure relates to a slide curtain coating apparatus and slide curtain coating method for applying coating liquid on a running web.
2. Description of the Related Art
Curtain coating apparatuses are commonly used in manufacturing processes of photosensitive materials such as photographic films.
FIG. 1 shows an example of a conventional curtain coating apparatus. The curtain coating apparatus 4 (or curtain coating head) includes one or more slits S as means for discharging a coating liquid 6. By discharging coating liquids 6 on the surface of a slide 1 from the multiple slits S, layers or curtain of the coating liquids 6 are formed on the surface of the slide 1. The laminate of the coating liquids 6, or curtain, then freely falls from the inclined surface of the slide 1 and contacts a web 5 running on a conveyor (not shown), forming a coating on the web 5. The curtain coating apparatus 4 further includes at least a pair of slide edge guides 2 and a pair of curtain edge guides 3. The present invention relates to an improvement of such a curtain coating apparatus.
The curtain coating method according to the present invention is directed to a method of forming a multilayer coating which involves the use of the aforementioned curtain coating apparatus, wherein coating liquids with different functions are discharged from different slits such that the coating liquids are stacked on top of each other on the slide surface to form a curtain, which then falls freely down on the running web to form a coating thereon.
A key issue in such conventional curtain coating apparatuses/methods is that, as shown in FIG. 1, the coating liquids flow slowly at the edges of the slide, i.e., as indicated by arrows A in the drawing, the flow rate of the curtain varies across its width, thereby causing a phenomenon in which edge flows converge to the slide center. As a result, the resulting coating has a greater thickness at the central region than at the edge regions. Due to such non-uniform thickness, the thicker edge regions of the coating may not completely dry in a drying process. This leads to “blocking” of the coating when it is rolled. And further, the raised edge regions can cause the web to be easily torn up when it is rolled. Thus, these drawbacks limit the efficiency of the curtain coating process. As a solution to overcome such problems, the drying temperature may be raised in the drying process; however, high drying temperatures are not desirable for the formation a for example, thermosensitive paper which develops colors upon exposure to high temperatures, leading to such problems as defective products. For this reason, this approach cannot be used for curtain coating in many cases.
Japanese Patent Application Laid-Open (JP-A) Nos. 2000-513, 2000-218209, 2001-104856 and 2005-512768 propose techniques for preventing the edge regions from becoming thicker. In the proposed techniques, an auxiliary liquid is allowed to flow essentially parallel to the flow direction of the curtain along edge guides of the slide, so that the flow rates at the opposing edges of the slide are made close to the flow rate at the center. The proposed techniques are disadvantageous in that, since a large amount of the auxiliary liquid needs to be supplied along the edges of the slide, the auxiliary liquid may be easily mixed with the curtain. And further, these technique have met with difficulties in stably and uniformly supplying the auxiliary liquid along the edge regions, resulting in non-uniform in thickness along its width. An additional disadvantage is that a complicated coating apparatus is required for this.